The two-stroke engine, also referred to as the two-cycle engine, has long been the power plant of choice for applications where power to weight ratio and mechanical simplicity are critical parameters for the operator. This is evident by their widespread use as outboard motors, motor-cross motorcycle racing engines and as the power plants for small, hand held tools such as chain saws and weed cutters.
Typical in these engines is a simple exhaust gas scavenging system established mainly by ports in the cylinder wall that are covered and uncovered by movement of the piston. Thus, numerous complicated and expensive seals, valves and related components required in four cycle engines are not required and are consequently omitted.
As the CAFE standards for automobile fleets have increased, the industry has placed even more of a premium on the power-to-weight ratio of the engine. A small engine of the same power as a larger one lowers the weight of the vehicle and permits the design of vehicles having smaller frontal area and consequent lowered wind resistance. Both of these design factors have beneficial effects on fuel economy.
While interest in two-stroke engines is very high in the automotive industry, the problem of unburned hydrocarbon emissions remains unsolved. Also, current and pending legislation on exhaust emission for off-highway vehicles, lawn and garden equipment and marine craft has brought the emission problems of the two-stroke engine to the forefront of those industries. Both the recreational and automotive industries are anxious for an economical way to control the emissions, in particular the unburned hydrocarbon emissions, and improve the fuel efficiency of two-stroke engines.
Numerous U.S. Patent discuss the construction, operation, and characteristics of two-cycle engines. Examples of such patents include: U.S. Pat. No. 4,399,788, issued Aug. 23, 1983, to Bostelmann for INTERNAL COMBUSTION ENGINE COMPRISING MEANS FOR CONTROLLING THE AXIAL EXTENT OF AN EXHAUST PORT IN A CYLINDER; U.S. Pat. No. 4,556,030, issued Dec. 3, 1985, to Aono for CONTROL ARRANGEMENT FOR INTERNAL COMBUSTION ENGINE; U.S. Pat. No. 4,576,126, issued Mar. 18, 1986, to Ancheta for TWO-STROKE INTERNAL COMBUSTION ENGINE; U.S. Pat. No. 4,903,648, issued Feb. 27, 1990, to Lassankse for ENGINE WITH IMPROVED EXHAUST GAS SENSING; U.S. Pat. No. 4,936,277, issued Jun. 26, 1990, to Deutsch et al. for SYSTEM FOR MONITORING AND/OR CONTROLLING MULTIPLE CYLINDER ENGINE PERFORMANCE; U.S. Pat. No. 4,960,097, issued Oct. 2, 1990, to Tachibana for AIR-FUEL RATIO CONTROL SYSTEM FOR TWO-CYCLE ENGINE; and U.S. Pat. No. 4,995,354, issued Feb. 26, 1991, to Morikawa for TWO-CYCLE ENGINE.
Although the large power to weight ratio of these engines is a desirable characteristic for automobile power plants, their high unburned hydrocarbon emissions (from short circuited air fuel mixture during the scavenging process) and the attendant fuel economy penalty has precluded their widespread acceptance into these markets. With respect to the high hydrocarbon emissions, it is known, for example, that hydrocarbon emissions of the typical two-cycle engine is many times higher (perhaps even ten times higher) than the level of similar emissions produced by the conventional four-cycle engine.
The massive quantity of unburned hydrocarbons discharged by the exhaust contribute greatly to inefficiency, wasted fuel, and pollution of the atmosphere. To some extent these problems have been ignored by continuing reliance on old technology or by choosing alternative power sources replete with their own inherent disadvantages such as higher cost, higher complexity and lower power-to-weight ratios.
In addressing the above-mentioned problems and operational characteristics in two-cycle engines, engineers and mechanics have experimented with a variety of structural components, seeking improvements and solutions. Typical carburetor and throttle devices vary the air/fuel ratio or the rate or directional path of air/fuel flow, or otherwise modify such variables as ignition, timing, or fuel composition.
A principal focus of researchers attempting to overcome known problems is the high degree of unburned hydrocarbons in the exhaust gas of two-cycle engines due to short circuiting of fuel during the scavenging process. Typically, the carburetor is adjusted to a selected air/fuel ratio, and then the flow of this mixture is throttled by an appropriate valve. In an outboard two-cycle engine the up-stroke of the piston creates a suction which draws in the mixture, the flow of which being throttled by partial blockage of flow into the crankcase.
One alternative control technique used in an engine is produced and is available under the commercial name "Orbital" (trademark; Orbital Engine Company Proprietary Limited, Balcatta, Australia). The operative principal underlying this system is the injection of fuel directly into the cylinder prior to combustion. More particularly, inlet air is pumped into the cylinder to scavenge or clean out exhaust gas. Later, as the piston rises and closes the inlet air port, fuel injection follows. In theory this should substantially eliminate unburned fuel from short circuiting since the scavenging air passing through the cylinder head does not carry the new charge of fuel. On the negative side of this design is the added work input of high pressure fuel injection directly into a closed cylinder head, as compared to the "Roots" blower (trademark; Dresser Industries, Inc., Dallas, Tex.) low pressure air flow (1 to 11/2 atmospheres) which carries the fuel into the cylinder via a typical simple and inexpensive carburetor. The air/fuel mixture in the Orbital control is varied by adjusting the high pressure fuel injection within the cylinder after the port is closed. To control such adjustments over a wide range is difficult, costly, and has not been proven satisfactory.